How are footings designed for axial and eccentric loads?

Short Answer:

Footings are designed for axial and eccentric loads by checking how the load is transferred to the soil safely. When the load is axial (central), the pressure is uniform across the base of the footing. But when the load is eccentric (off-center), it creates bending moments, causing uneven pressure on soil—more on one side and less on the other.

In such cases, footing design includes checking for maximum soil pressure, moment due to eccentricity, and reinforcement to resist bending. This ensures that the footing remains safe, stable, and does not tilt or crack under uneven loads.

Detailed Explanation:

Footings designed for axial and eccentric loads

In RCC structure design, footings are crucial as they transfer the load from columns to the ground. These loads may be axial (vertical) or eccentric (not aligned with the center of the footing). While axial loads are easier to handle, eccentric loads require special design considerations because they create additional bending and non-uniform soil pressure.

Understanding the behavior of footings under different loading types helps in designing a foundation that is both safe and economical. The main goal is to avoid soil failure, structural cracks, or uneven settlement.

Design for Axial Load

When the axial load is applied, it passes through the center of gravity of the footing. The soil pressure under the base remains uniform, and the design is simpler.

Key design steps:

  • Calculate axial load (P) from the structure.
  • Estimate safe bearing capacity (SBC) of soil from site data.
  • Determine required area of footing = P / SBC.
  • Choose footing shape (square, rectangular, circular) to match required area.
  • Design reinforcement to resist bending due to soil reaction (downward pressure).

Axial load causes direct compression, and footing design is based on bending moment due to soil pressure acting upwards, balanced by the column load acting downwards.

Design for Eccentric Load

When the load is not centered, it causes eccentricity (e), which results in both axial load and bending moment on the footing. This makes the soil pressure non-uniform—it increases on one side and reduces on the other.

Key considerations:

  • Eccentricity (e) = M / P, where M is the moment and P is axial load.
  • Soil pressure is calculated as:
    • q = P/A ± M/Z
    • Where A is area, Z is section modulus.
  • Check for zero tension condition (no uplift or gap between footing and soil).
  • Adjust the footing size or shape to keep pressure within safe limits.
  • Provide more reinforcement in the direction of bending to resist moment.

If eccentricity is large, it can even cause partial uplift on one side of the footing, which must be prevented by increasing footing width or adding counteracting features.

Practical Methods in Eccentric Load Design

  • Use combined footings when one column is near a property line.
  • Use strap footings to connect two individual footings and balance the moment.
  • For isolated footings with minor eccentricity, use unequal reinforcement distribution.
  • Increase depth or thickness of footing to resist additional moments.

Designs are verified using IS 456:2000, and the final footing must resist all moments and pressures without exceeding soil capacity or failing structurally.

Conclusion:

Footings under axial loads are designed for uniform pressure and bending due to upward soil reaction. Footings under eccentric loads are more complex—they involve combined axial and bending action, resulting in uneven soil pressure. These require extra checks and reinforcements. Proper footing design ensures that the structure remains stable, safe, and performs well during its life.